## Tuesday, January 19, 2010

### Chapter 2 - Exercise 10 Part 2

Today's post concludes Exercise 10 - this is the second half of the exercise and involves building the circuit shown on page 81 and taking some readings of the amps flowing through two different locations on the breadboard.

First off... this exercise was very frustrating. Not because the circuit is incorrect, but because it took me forever to figure out that my cheapy, non-ranging multimeter wasn't up to the task of providing me with any readings... only after switching to my autoranging (and manually switching it to mA reading) did I start to get useful readings.

Once again, I had to modify the circuit a bit because I lacked those pesky 180 ohm resistors... I just substituted 220 ohm resistors in their place. I didn't get the readings indicated in the book, but that makes sense given that I've put more resistance in the circuit.

You'll have to look close at my photos to figure out which is which - you can use the location of where I place the probes to match them up to the proper location (A1 or A2). This is where having those patch wires is really handy - I used a bunch of them to connect the potentiometer to various components... I also used small pieces of stripped 22 gauge wires as places to touch the probes... again, look closely and you should see how I did it.

My maximum A2 reading (with the potentiometer wide open) was around 16mA... reading at A1 (potentiometer wide open) was .53mA. These readings are very odd, but then again, my circuit has some variations in resistance AND my AC Adapter is putting out about 12.25 volts... and it's Tuesday. Weird things happen on Tuesdays.

Anyway, on to Exercise 11...

1. I had a similar problem with my auto-ranging meter on exercise 4. The thing still reports the wrong number when in the circuit. I can fake it by putting the thing together in 10amp mode, and looking at the 3 decimal place. I bought a non-ranging one to get past that problem.

2. I get very different readings from those the author gets. Using a 1K and a 220 ohm resistor in parallel gives you 180 ohms of resistance, so I have the resistance right. Here's the series:

A1 : my A2 : his A2
.01 : 0.37 : 1.9
.02 : 2.09 : 4.9
.03 : 3.65 : 7.1
.04 : 6.13 : 9.9
.06 : 8.45 : 12.9
.08 : 11.81 : 17.9
.09 : 13.44 : 22.1

His emitter current is consistently 50% more than mine. Why?? Is this a function of the transistor? The potentiometer? It seems like this is important. Don't we need to be able to predict the current?

On a side note, he says his base/emitter ratio is about 24:1 but it looks to me like he's off by an order of magnitude. If A1=.05 and A2=12.9, then 12.9/.05 = 258

3. I did this yesterday, and also had trouble getting readings from A1 - ours were super tiny.

4. Just did this experiment. Malcontent is correct: The transistor amplifies the current by a factor of ~240 (not 24, as printed in the book). This is confirmed in the book's errata:
http://oreilly.com/catalog/errata.csp?isbn=9780596153748&print=yes

I had a little trouble with this, because I didn't pay attention to the text about the transistor's saturation point of about 0.12mA. The author tests 0.01 - 0.12mA, but I tested a range of 0.05 - 0.55mA (which was the range my potentiometer passed through). What do you know, for all my readings except 0.05 and 0.10, I got roughly the same reading from the emitter (~34mA). Took me a while re-reading to realize my error.

If I had a second multimeter, I'd re-test the 0.01-0.12mA ranges, but it's kind of a drag moving the meter between points A and B for each reading. I think I understand enough about the intent of the experiment to call it 'done' and move on.